Ab initio and DFT studies on methanol-water clusters.

The gas-phase geometries, binding energies (BEs), vibrational spectra, and electron density topological features of methanol (M), water (W), and methanol-water mixed clusters (M(m)W(n), where m = 0-4 and n = 0-4; m + n < or = 4) have been calculated using Hartree-Fock, second-order Møller-Plesset perturbation, and density functional theory with Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional methods. Bader's "atoms in molecules" theory has been used to analyze the hydrogen bonding network. To understand the effect of cooperativity, we have performed natural bond orbital analysis and reduced variational space decomposition analysis. The results show that BEs of methanol and mixed clusters are higher than those of water clusters due to the electron-donating nature of the methyl group. These findings are in accordance with the role of cooperative polarization and cooperative charge transfer in the methanol and mixed clusters. As the size of the cluster increases, the contribution from the cooperative effects also increases. The cooperativity contributes approximately 14 and 24% of stabilization in trimers and tetramers, respectively. The calculated nu(OH) frequencies at MP2/6-311++G(d,p) are in close agreement with the corresponding experimental values.